The invention relates to a device for measuring the position of an object that can be moved forward and backward along a fixed path, such as a closure element of an open roof construction for a vehicle. The device includes a driving unit for said object and a pulse generator, which is driven by said driving unit. The pulse generator delivers pulses during the movement of the object along said path.
In the case of an object that can be moved forward and backward along a fixed path it is often desirable to know the position of the object along said fixed path. In the above example of a closure element of an open roof construction for a vehicle, the measured position of the closure element can be communicated to the user, so that the latter is informed of the current position of the closure element. The number of pulses delivered during the movement of the object is counted by a suitable processing unit and converted into a position indication that can be recognized by the user.
With prior art devices for measuring the position of an object of the present type that can be moved forward and backward along a fixed path there is a risk of an incorrect number of pulses being delivered. The number of pulses in question may either be too large or too small. The related problems can again be explained by means of the above example. When a closure element of an open roof construction is moved from the closed position to the fully open position by the user, the intention is to obtain, by measuring the position of the closure element, an indication when the closure element has reached a specific position, such as the fully open position in this case. When the pulse generator has delivered too few pulses during the movement of the closure element, the user is incorrectly informed as to the current position of the closure element. While the closure element has already reached the fully open position, the user will think, on the basis of the information that he has received, that the closure element is not fully open yet. Consequently the user will excite the driving unit for the closure element to move the closure element further in the opening direction, while such a movement is no longer possible. This may cause an overload of the driving unit (for example an electric motor in the aforesaid example).
Comparable problems will arise in the case that too many pulses are delivered by the pulse generator.
Apart from that, it is noted that it is not necessarily the pulse generator that causes the problem of too many or too few pulses being delivered. For instance, failures in electronic components and the like can lead to an incorrect number of pulses, as a result of which the distance over which the object has been moved along the fixed path is no longer in correct agreement with the corresponding number of pulses.
In view of the problem indicated in the foregoing it is an object of the present invention to provide a device for measuring the position of an object that can be moved forward and backward along a fixed path, by means of which the position measurement in question can be monitored.
In one embodiment, the device includes a slow potentiometer or similar device, which potentiometer is likewise driven by the driving unit and which is connected to the driving unit via a transmission, such that the potentiometer passes through its range of values (e.g. resistance) maximally once during a complete movement of the object along its fixed path.
During movement of the object along the fixed path, a determination of the position thereof takes place, which is based both on the number of pulses delivered by the pulse generator and on the signal from the slow potentiometer. When the measured number of pulses deviates too much from the theoretically correct number of pulses, a discrepancy will occur between the position signal based on the number of pulses and the position signal based on the slow potentiometer. In such a case, an error signal or the like can be delivered. In the aforesaid example, the user can, for example, be informed that the system must be reset.
The term xe2x80x9cslow potentiometerxe2x80x9d is used to express that this potentiometer passes through its range of values maximally once during the complete movement of the object along its fixed path. Commonly, this means that the potentiometer is turned through 360xc2x0 at most. In this way there is an unequivocal relationship between the signal delivered by the potentiometer (e.g. voltage) and the position of the object along said path. The determination of the position of the object along the path on the basis of the slow potentiometer is less accurate than on the basis of the number of pulses delivered by the pulse generator, of course, but this does not constitute a problem, since the signal delivered by the slow potentiometer is only used for signalling a deviation that exceeds a specific limit (in the present case this means that the number of pulses being delivered is a predetermined number of pulses higher or lower than the theoretically correct number of pulses) As long as no such signalling takes place, the number of pulses delivered by the pulse generator forms the basis for an accurate measurement of the position of the object.
Within the scope of the invention there are several possibilities for realising various embodiments thereof. According to one variant, for example, the pulse generator comprises a quick potentiometer that does not have limit stops, which potentiometer passes through its range of resistance a large number of times during a complete movement of the object along its fixed path. In this case the potentiometer is called a quick potentiometer, in contrast to the aforesaid slow potentiometer, since it passes through its range of resistance several times rather than only once during a complete movement of the object along its fixed path. This directly implies that the quick potentiometer commonly does not have limit stops, that is, it should be capable of multiple or even unlimited rotation without reaching a limit stop during the complete movement of the object. In one embodiment, each complete revolution of the quick potentiometer corresponds to one pulse. Within each pulse a variation of the signal (e.g. voltage) delivered by the quick potentiometer takes place, so that a highly accurate position measurement is possible on the basis of the signal from the quick potentiometer.
In another embodiment, the pulse generator is a digital pulse generator, such as a slotted disc that is rotated by the driving unit, which disc cooperates with a photocell arrangement. During each rotation of the disc a number of pulses (generally in block form or a square wave) corresponding to the number of slots is delivered. With such an embodiment, there is no variable signal within one pulse, so that the position measurement only takes place on the basis of the number of pulses rather than on the basis of the position of a respective pulse, as is the case with the preceding embodiment
Finally, an advantageous embodiment of the device according to the invention is one wherein the slow potentiometer does not have limit stops. The advantage that is thus achieved is that it is not necessary for the slow potentiometer to move from its minimum value (e.g. resistance value) position (minimum voltage) to its maximum value position (maximum voltage), or vice versa, during a movement of the object from one end position to the other end position. During said movement of the object, it is now also possible for the slow potentiometer to move from a specific position having a specific value to its maximum value position, then reach the position wherein the value drops to a value such as zero and subsequently moves to substantially the position having the initial value. Also in this case the potentiometer passes through its range of values maximally once, but a zero passage or drop is included. Also here there is an unequivocal connection between the signal delivered by the slow potentiometer and the position of the object.
The use of a slow potentiometer not having limit stops provides benefits during mounting operations, since it is not necessary to mount the slow potentiometer in a specific starting position.